MPEG is a digital data transfer standard, and is well known as a standard for digital compression of video signals and audio signals. An MPEG2 data stream of digitally compressed video signals and audio signals is distributed across a network. A decoder at a reception location receives the data stream and reproduces video and audio signals. The speed at which a data stream is distributed across a network must be high because a large amount of data for video signals and audio signals are transmitted.
A transmission speed of about 750 kbytes per second is required in order to attain the high resolution and picture quality needed for the general TV broadcasting. In this situation, bidirectional communication is not easy because of the transfer speed involved. Thus, the transfer of data by CATV and broadcast satellites is effected by employing unidirectional communication within a band area in which the data are to be transmitted. For an adequate exchange of data when employing unidirectional data distribution, sync data called program clock reference (hereinafter referred to as "PCR") is included in the MPEG2 data stream emitted by the transmission source. The decoder extracts the PCR sync signal from the received data stream to obtain the sync data originating at the transmission source. The PCR is time data that is loaded during the encoding of data, and is processed at the transmission source during the transmission.
The decoder includes a system time clock (hereinafter referred to as an "STC"), which is sync data available at the decoder, and which is generated based on a reference clock when decoding is performed by the decoder. The decoder detects the PCR in the received data stream, and compares the detected PCR with its internal sync data STC. It is possible from the difference between the two to identify the degree of transfer shift for the reference clocks between the transmission side an d the reception side. If the sync data differ, it means that the transfer speeds identified by the transmission source and the reception location differ. When there is a difference in their identified speeds, a data overflow may occur at the decoder, or reproduced data may be cut off. Thus, the decoder updates the STC and synchronizes it with the PCR data to acquire a reference clock having a frequency for which synchronization for transfer can be acquired.
A PLL (Phase Locked Loop) structure, including a voltage control oscillator, is employed as a circuit for acquiring such synchronization. The voltage control oscillator can vary an output frequency (clock) in accordance with a received voltage. A first PCR extracted from a specific data stream is set to a counter, and the value held by the counter is sequentially incremented in accordance with a change in the pulse, which is the output (i.e., the reference clock) of the voltage control oscillator. That is, the value currently set in the counter is the current sync data STC of the decoder. The STC is compared by the feedback control with the PCR extracted from the received data stream. A control voltage that corresponds to the difference between the data is supplied to the oscillator, thereby updating the frequency of the reference clock in the decoder. When the STC matches the PCR, i.e., is in the locked state, the control of the oscillator is stabilized. Such a control sequence is called a clock recovery operation.
If a plurality of short data streams are sequentially distributed in a short period of time, the next new data stream may be distributed before the control by the voltage control oscillator in the decoder is stabilized. In this case, the process whereby the STC is locked to the PCR must be repeated from the beginning by the decoder. For MPEG2, the frequency of the reference clock at the decoder is initially set to 27 MHz, for example, and is updated to an optimal value in accordance with the difference between the STC and the PCR. The frequency is stabilized when the STC and the PCR match. When distribution of a new data stream is detected, the frequency of the reference clock at the decoder is reset to the initial value of 27 MHz, and its updating begins again at this value. If the reference clock of the transmission source has a higher frequency than that at the decoder, received data are accumulated in a buffer at the decoder during a period that continues until the control of the oscillator is stabilized. As a worst case, the quantity of data may increase until it exceeds the buffer capacity, resulting in the loss of data.
Such a case may occur in commercial base broadcasting. For example, when a data stream is distributed wherein the same advertisement is inserted into it many times, and the stream for a single commercial is employed a number of times. In this case, each time distribution of the commercial data stream is made, the PCR is reset and returns to zero.
In accordance with the MPEG standards, 10 or more transfers per second are required for the sync data PCR originating at the transmission source. Accordingly, the decoder must compare the STC with the PCR at least 10 times per second. Since generally for the dispersion feedback control about 20 to 100 samples are required before stabilization is realized (under servo), a period will be needed of around two seconds, at least, from the start of the control process until the locked state is attained. Therefore, when another data stream is distributed in a shorter period of time, the above described problems would occur.
It is believed that the distribution of the MPEG2 transfer stream will become primary in the commercial base. It is probable that short data streams, such as those for commercials, may be sequentially distributed in a short period of time.
It is, therefore, one object of the present invention to provide a method for smoothly acquiring synchronization, even when short data streams are received in a short period of time.
It is another object of the present invention to prevent, to the extent possible, the accumulation of data in the buffer of a decoder, even when short data streams are sequentially received.
It is an additional object of the present invention to prevent the occurrence of data loss during data reproduction.